Anodic stripping voltammetric apparatus

ABSTRACT

In anodic stripping voltametric apparatus the time constant of the filter circuit of the voltage holder is selected to be of from 0.01 to 0.1 second for the purpose of analyzing an extremely small quantity of metal ions at high accuracies.

BACKGROUND OF THE INVENTION

This invention relates to voltammetric apparatus, more particularlyimproved apparatus for use in differential pulse anodic strippingvoltammetry (hereinafter abbreviated as DPASV).

DPASV is recognized in the art as an effective method for analyzing aminute quantity of components, especially metals contained in solutions.

The method of DPASV consists essentially of the following two steps. Inthe first step a plating voltage about several hundreds millivoltsnegative with reference to the oxidation reduction potential of thecomponent to be analyzed is impressed upon a working electrode immersedin a solution to be measured for plating the working electrode. In thesecond step a voltage comprising direct current voltage (strippingvoltage) which increases gradually with time in the positive directionfrom the plating voltage and superimposed upon a pulse voltage isapplied by sweeping upon the working electrode for stripping thecomponent that has been plated on the working electrode during the firststep. For this reason, the second step is termed the stripping step.FIG. 1 of the accompanying drawing is a graph showing the manner ofapplying the voltage in the first and second steps. Although in FIG. 1the pulse voltage is shown as being superimposed upon the strippingvoltage (sweep potential) on the positive side thereof, it is alsopossible to superimpose the pulse voltage on the negative side of thestripping voltage.

In sweeping during the second step, among the current flowing throughthe working electrode, the current that flows while the pulse is notapplied, preferably the current that flows during definite interval(sampling time S₁) immediately prior to the application of the pulsevoltage, and the current that flows while the pulse is applied,preferably a definite interval of the latter half of the pulse (samplingtime S₂) are measured to obtain sampling currents IS₁ and IS₂respectively and the difference between these sampling currents isdetermined. FIG. 2 diagrammatically shows one example of therelationship between the pulse and the sampling times S₁ and S₂.

To calculate the difference between sampling currents IS₁ and IS₂, thesecurrents are usually converted into corresponding sampling voltageswhich are held by respective voltage holders and then the differencebetween these sampling voltages is determined. The maximum value or theintegrated value of the voltage difference is used to calculate theconcentration of the component to be analyzed.

In this case curves representing sample currents IS₁ and IS₂ areillustrated in FIG. 3. As has been described above where the pulsevoltage is superimposed on the negative side of the stripping voltagecurve S₂ will be positioned beneath curve S₁ with its polarity reversed.The difference between sample currents IS₂ and IS₁, that is the outputsignal is shown by FIG. 4.

The outline and present state of DPASV can be found in the followingpapers.

a. J. B. Flato, Analytical Chemistry, Vol. 44, September, 1972, pages75A-87A.

b. H. Seigerman et al., Americal Laboratory, Vol. 4, No. 6, pages 59-68(1972)

c. T. R. Copeland et al., Analytical Chemistry, Vol. 46, No. 14, Dec.1974, pages 1257A-1264A.

One example of the apparatus for use in DPASV is described in U.S. Pat.No. 3,420,764.

Although DPASV is an analytical method having an extremely highsensitivity, it is desired to improve further the sensitivity.

Thus, the prior art apparatus for use in DPASV has such problems as poorseparation of the peaks obtained on a recording paper, asymmetricalshape of the peaks and low measuring accuracies of various components.Moreover, both ends of the base line of the peaks recorded on arecording paper tend to shift upwardly thus forming a dish like baseline. Consequently, the measuring accuracy of the peaks of certaincomponents such as copper and zinc appearing on both ends decreases. Theoutput signal obtainable in such a case is illustrated in FIG. 5.

As a result of our investigation we have found that these difficultiescan be improved greatly by improving the filter circuit utilized in thevoltage holding circuit.

SUMMARY OF THE INVENTION

It is an object of this invention to provide improved apparatus for usein differential pulse anodic stripping voltammetry (DPASV) capable ofanalyzing an extremely small quantity of components in solutions at highaccuracies.

Another object of this invention is to provide improved apparatus foruse in DPASV capable of analyzing ions of various metals with peakssufficiently spaced and a flat base line.

According to this invention there is provided apparatus for use indifferential pulse anodic stripping voltammetry of the class comprisingan electrolytic cell provided with a working electrode and a counterelectrode; a source of plating voltage, a source of sweeping strippingvoltage and a source of pulse voltage for impressing plating voltage,stripping voltage and pulse voltage respectively across the workingelectrode and the counter electrode; a current-voltage converter forconverting the current flowing through the working electrode intovoltage for producing two sample voltages; voltage holding means forholding the two sample voltages; and a differential amplifier foramplifying the difference between the two sample voltages held by thevoltage holding means, wherein said voltage holding means includes afilter circuit having a time constant of from 0.01 to 0.1 second.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram showing one example of the method of applyingvoltages in DPASV:

FIG. 2 is a diagram showing the relationship between a pulse andsampling time;

FIG. 3 show one example of curves showing sample currents;

FIG. 4 is a graph showing an output signal curve;

FIG. 5 is a graph showing sample current wherein the base line is curvedupwardly and the seperation between peaks is not sufficient;

FIG. 6 is a graph showing improved output signal obtainable by theapparatus of this invention;

FIG. 7 is a block diagram showing one example of the construction of theapparatus of this invention;

FIG. 8 is a side view, partly in longitudinal section, showing oneexample of the hanging mercury drop electrode utilized in thisinvention;

FIGS. 9 and 10 are longitudinal sectional views showing two examples ofthe solid electrode;

FIGS. 11 and 12 are connection diagrams showing two types of the outputcircuit utilized in this invention;

FIG. 13 is a graph showing the relationship between the pulse repetitionperiod, and the relative sensitivity by utilizing the sweeping speed asa parameter;

FIG. 14 is a block diagram showing a modified embodiment of thisinvention wherein the input terminals of voltage holders are groundedthrough switches;

FIG. 15 is a plan view of a measuring cell utilized in this invention;

FIG. 16 is a rear view of the measuring cell shown in FIG. 15;

FIG. 17 is a cross-sectional view of the cell shown in FIG. 15 takenalong a line XVII--XVII;

FIG. 18 is a sectional view taken along a line XVII--XVIII in FIG. 16;

FIG. 19 is a sectional view of the cell shown in FIG. 18 with a workingelectrode mounted thereon and

FIG. 20 is a sectional view of the cell shown in FIG. 16 taken along aline XX--XX.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The detail of the basic construction of the apparatus of this inventionwill now be described with reference to the block diagram shown in FIG.7, in which reference numeral 1 designates an electrolytic cellcontaining a solution to be measured and a working electrode 2 and acounter electrode 3 immersed therein. Further, a reference electrode 4is also disposed in the cell 1 for compensating for the effect caused bythe variation in the concentration of a component to be analyzed in theliquid being measured. Reference electrode 4 is connected to an additioncircuit 9 via an impedance converter 8.

A plating voltage source 6 and a sweeping stripping voltage source 7 areconnected to the counter electrode 3 respectively through contacts SW₄and SW₅ of a transfer switch 5 and the addition circuit 9. A source ofpulse 10 constructed to produce a DC pulse having an amplitude of 10 to100 millivolts, for example, is connected to the counter electrode 3 viacontact SW₃ of a timing switch 11 and the addition circuit 9. The pulsehaving a duration or width of 8.3 to 100 milliseconds determined by theopening and closing of the contact SW₃ is impressed across the workingelectrode 2 and the counter electrode 3. The waveform of the pulse ispreferably to be rectangular or similar shapes.

The shorter is the pulse period (pulse repetition period), the higherare the sensitivity and speed of analysis, a preferred pulse periodranging from 16.7 to 400 miliseconds, more particularly from 33.4 to 200milliseconds. A preferred sweeping speed of the stripping voltage rangesfrom 20 to 60m V/sec. FIG. 13 shows a group of curves showing therelationship between the pulse period and the relative sensitivity byusing various sweeping speeds as a parameter.

The working electrode 2 is connected to voltage holding means includingtwo voltage holders 13 and 14 respectively through a current-voltageconverter 12 and contacts SW₁ and SW₂ of the timing switch 11. The twosample voltages held by voltage holders 13 and 14 are applied to adifferential amplier 15 to produce an output signal.

The detail of respective elements described above will now be described.

Any type of the electrolytic cell 1 may be used provided that it issuitable to contain the liquid to be measured. For example, cylindricalor inverted cone shaped container made of glass, plastics, etc. may beused.

Among the working electrodes 2 that can be used in this invention areincluded a hanging mercury drop electrode (HMDE), an electrode made ofplatinum, nickel, tungsten, graphite or glassy carbon, an electrodeprepared by moulding a mixture of a carbon powder and a hard polymer,epoxy resin for example, a carbon paste electrode prepared by moulding amixture of a carbon power and paraffin, a mercury film electrodecomprising a solid electrode coated with a mercury film, etc.

Among these electrodes, the hanging mercury drop electrode (HMDE) isespecially suitable for use as the working electrode of the apparatus ofthis invention. In principle, this electrode comprises a capillary tubehaving a mercury pool and a microhead connected to the capillary tube.Various types of HMDE are known in the art and any one of them can beused in this invention.

Although HMDE is suitable, when the temperature of the mercury variesduring use, the surface area of the mercury drop varies thus degradingthe reproduceability of the result of measurement. Further, presence ofair bubbles in the mercury pool makes it difficult to adjust the size ofthe mercury drop. Accordingly, it is essential to maintain the roomtemperature at a constant value during the use of the HMDE. Moreover, itis necessary to take care not to entrain air bubbles in the mercurywhile pouring the same.

The following HMDE from which defects described above have beeneliminated is especially suitable for use in this invention wherein acapillary tube provided with a mercury pool is connected with amicrohead through a joint in the body of the electrode and constanttemperature cooling water is circulated about the mercury pool, asillustrated in FIG. 8. When a knob 21 of the microhead is rotated, aplunger 23 is reciprocated in the longitudinal direction through a feedscrew rod 22. The microhead is connected to the main body 25 through ajoint 24. The lower end of the joint 24 is connected to a mercury pool29 via a packing 26. A space 30 for circulating cooling water surroundsthe mercury pool 29. The cooling water maintained at a constanttemperature is admitted into the space 30 through an inlet port 31 anddischarged through an outlet port 28 as shown by arrows. A hollowcapillary tube 37 extends downwardly from the mercury pool 29 and amercury drop 38 is formed at the lower end of the capillary tube. Anelectrode terminal 40 is connected to the joint 24. The capillary tube37 is provided with a flange 39 at its upper end and inserted into thecentral bore of the main body 25. O-rings 27 and 33 are interposedbetween the capillary tube 37 and the main body 25 for sealing the space30. The O-ring 33 is maintained in position by a stationary screwbushing 35 at the lower end of the main body. An O-ring 36 is providedto seal the gap between the capillary tube 37 and the stationary screwbushing 35 whereas O-rings 32 and 34 are provided for mounting theelectrode on the electrolytic cell. With this construction, since thetemperature of the mercury pool is maintained at a constant value it isnot necessary to maintain the temperature of the measuring room at aconstant value. Where the main body is made of transparent material suchas acylic resin it is possible to readily confirm the presence orabsence of air bubbles in the capillary tube. Moreover, as the variousparts of the electrodes are interconnected through the joint, packingand O-rings their assembling and disassembling are easy.

As has been described above, a solid electrode made of metal or carboncan also be used as the working electrode of this invention. Although asolid electrode is easier to handle than a mercury electrode, as thesurface condition of the electrode varies during use therebydeteriorating the reproduceability it is necessary to subject theelectrode to a regeneration treatment requiring substantial labour andtime.

A solid elecrode prepared by moulding a mixture of a powder ofelectroconductive substance and a soft polymer is free from thesedifficulties. As the powder of the electroconductive substance ispreferred a powder of carbonaceous substance such as carbon black,graphite and glassy carbon but a powder of such metals as platinum,gold, silver, copper, etc. can also be used. Preferably, the particle isrelatively fine, a preferred average particle diameter being less thanabout 10 microns, more preferably from about 0.05 to 5 microns. As thesoft polymer may be used silicone rubbers, fluorinated rubber,polyethylenes, neoprenes, natural rubber, polysulfide rubber, nitrilerubber and so forth. Silicone rubbers are especially suitable. Preferredhardness of the soft polymer is about HS 30 to HS 90 measured by aspring according to JISK6380, more preferably from about HS 40 to HS 80.Although the percentage of the electroconductive substance differsdependent upon the electroconductivity and particle size thereof, in thecase of a carbon powder the percentage usually ranges from 30 to 80% byweight, preferably from 30 to 60%, whereas in the case of a metal powderfrom 10 to 80%, more preferably from 20 to 60%, by weight. The pointedsurface of the electrode made of conductive substance can readily beregenerated by cutting it with a knife blade. FIGS. 9 and 10 arelongitudinal sectional views showing examples of such solid electrode.

FIG. 9 shows an example of a solid electrode comprising a rod shapedelectrode 41 having a circular or square cross-section and made of anelectroconductor and an insulating tube 42 covering the electrode. Theinsulating tube comprises a heat shrinkable tube made of Teflon,silicones or polyethylens. It is advantageous that such insulating tubeshould be soft and can be cut readily for regenerating the electrodesurface. The covered electrode rod 41 snugly fits in the opening at oneend of a cylindrical or square electrode holder 43 made of glass,plastics or metal. To ensure snug fit the inner diameter of theelectrode holder 43 is made to be slightly smaller than the outerdiameter of the insulating tube or a binder may be used at the joint. Ifdesired, the joint between the electrode holder 43 and the insulatingtube 42 may be covered by a heat shrinkable tube. A cap 44 is threaded,force-fit or bonded to the upper end of the electrode holder 43 and alead conductor 45 extends through cap 44 to be received in an opening inthe upper end of the electrode rod 41.

FIG. 10 shows an example of a composite electrode wherein an electroderod 51 is contained in an electrode cylinder 56 via an insulating tube52. The upper end of the electrode cylinder 56 is snugly held by thelower end of electrode holder 53 via an insulating tube 57. Leadconductors 55 and 58 extend through cap 54 to be received in theopenings at the upper ends of the electrode 4od 51 and the electrodecylinder 56, respectively. In the composite electrode of this type, thebottom surface of the electrode rod 51 is used as the working electrodewhereas the outer periphery of the electrode cylinder 56 as the counterelectrode having a large surface area.

Except the hanging mercury drop electrode, the working electrode 2 maybe either a stationary type or a rotary type. The counter electrode 3may be made of platinum, tungsten or carbon, for example, whereas as thereference electrode may be used a calomel electrode, a sliver-silverchloride electrode, etc.

The construction and operation of the source system including platingvoltage source 6, sweeping stripping voltage source 7 pulse voltagesource 10 and adder 9, and in some case further including referenceelectrode 4 and impedance converter 8; the switch system includingplating-stripping transfer switch 5 and timing switch 11; the outputsystem including current-voltage converter 12, voltage holders 13 and 14and differential amplifier 15 are well known in the art so that theirdetailed description is believed unnecessary.

For example, each one of the plating voltage source 6 and the pulsevoltage source 10 may comprise a battery and a potentiometer. Thesweeping stripping voltage source 7 may comprise an integrating circuitconstituted by an operational amplifier, capacitor and a resistor, andthe impedance converter 8 may comprise a voltage follower circuitutilizing an operational amplifier, for example. As the addition circuit9 may be used an adder including an operational amplifier. Theplating-stripping transfer switch 5 may comprise a reed switch, forexample.

The timing switch 11 may comprise a semiconductor analogue switch whilethe current-voltage converter 12 may comprise a current-voltageconversion circuit including an operational amplifier, for example. Thevoltage holders 13 and 14 may be constituted by operational amplifiers.The differential amplifier 15 may comprise an operational amplifier, forexample.

These circuits operate to produce an output according to the followingequation.

    Output = IS.sub.2 - IS.sub.1

if a correction term is used for this mathematical operation, a circuitfor operating the correction term is added to the differential amplifier15.

FIG. 11 shows one example of the output circuit system of the apparatusof this invention wherein 2, 11 through 15 designate correspondingelements shown in FIG. 7. More particularly, working electrode 2 isconnected to the input terminal of the current-voltage converter 12 andthe output terminal thereof is connected to the input terminal of thetiming switch 11. The circuit including a capacitor 62 grounded throughresistors 64 and 65 comprises a so-called roll of circuit and is usedfor eliminating noise. Capacitor 66 is provided for the same purpose.

The output from an operational amplifier 63 comprising a portion of thecurrent-voltage converter 12 is applied to inputs A and B of voltageholders 13 and 14 respectively through switch contacts SW₁ and SW₂. Acircuit comprising resistor 68 and capacitors 67 and 69 and connectedbetween input terminal A and an operational amplifier 73 of the voltageholder 13 comprises a π type filter and the voltage is held by capacitor69. The output from the operational amplifier 73 is fed back to aninput - for forming the so-called voltage follower. This output is alsoconnected to one end of resistor 75 of the differential amplifier 15. Inthe same manner, the input terminal B of the voltage holder 14 isconnected to one end of resistor 76 of the differential amplifier 15 viaa π type filter including resistor 71 and capacitors 70 and 72, and anoperational amplifier 74.

The other ends of resistors 75 and 76 of the differential amplifier 15are connected to - and + input terminals, respectively, of theoperational amplifier 79 whose output is fed back to the + inputterminal via resistor 78. The + input terminal of the operationalamplifier 79 is grounded via resistor 77 to form a differentialamplifier circuit.

Among various elements of the output circuit system shown in FIG. 11,capacitors 67 and 70 are used to eliminate high frequency components.The circuit including operational amplifier 74 and resistors 76 and 77is convinient to check the operation of the circuit but these circuitelements may be omitted, as shown in FIG. 12.

The invention is characterized in that the time constant of the filtercircuit of the voltage holder of the apparatus thus far described isselected to be in a range of from 0.01 to 0.1 sec. With reference toFIG. 11, it is possible to improve the separation of adjacent peaks andto flatten the base line by making the product of the resistance andcapacitance or the time constant of the filter circuits respectivelyconstituted by resistor 68 and capacitor 69, and resistor 71 andcapacitor 72 of the voltage holders 13 and 14 to be equal to 0.01 to 0.1second, preferably 0.01 - 0.08 second, more preferably 0.01 -0.05second, an appropriate value being selected according to theelectroconductivity of the solution being measured.

FIG. 6 shows a curve of the output signal obtainable by the apparatus ofthis invention. By comparing the curves shown in FIGS. 5 and 6 it can benoted that the separation of the peaks has been improved and the baseline flattened.

Better results can be obtained when the value of resistor 61 of thecurrent-voltage converter 12 is selected to be in a range of 100 to 500K ohms, that of resistor 64 in a range of 0.02 to 1 K ohms, that ofresistor 65 in a range of 5 to 10 Kohms, the capacitance of capacitor 62in a range of 500 to 10⁵ PF and that of capacitor 66 in a range of 10³to 10⁵ PF.

A preferred embodiment of this invention has been described in detailhereinabove, but the characteristics of the apparatus can be improvedfurther by the following modifications.

Where measurements are made sequentially by using the apparatus of thisinvention, after completion of one measurement the voltage held by thevoltage holders affects the accuracy of the next analysis. This problemcan be solved by discharging the voltage held by the voltage holderseach time when a measurement has completed, for example by grounding theinput terminals of the voltage holders through switches.

FIG. 14 is a block diagram showing one example of such modificationwherein numerals 1 through 15 represent the same elements as those shownin FIG. 7. As shown, the input terminals of the voltage holders 13 and14 are grounded through contacts SW₆ and SW₇ of timing switch 81 whichmay be a semiconductor switch (analogue switch) as before. Duringmeasurement, contacts SW₆ and SW₇ are opened but closed upon completionof the measurement for discharging the voltage held by the voltageholders. Although closing and opening of these contacts may be mademanually, it is advantageous to operate them by means of an automaticsequence circuit which as is well known in the art is usuallyconstituted by relays, timers and logic elements, integrated circuits,for example.

To analyze according to DPASV, usually inert gas, such as nitrogen orhydrogen is firstly blown into the solution being analyzed and containedin the electrolytic cell for removing oxygen dissolved in the solution(oxygen removing step). Thereafter, the plating voltage is impressedupon the working electrode for plating or depositing the component to beanalyzed (plating step). Then, the sweeping stripping voltage and pulsevoltage are applied to strip the component to be analyzed. (strippingstep). During the oxygen removing step and the plating step describedabove, for the purpose of improving the efficiency, the solution in theelectrolytic cell is usually stirred.

One may expect that the effficiency can be improved by increasing thestirring speed. Actually, however, it is advantageous to stir at arelatively low speed during the oxygen removing step and at a relativelyhigh speed during the plating step. Where a high speed stirring iseffected during the oxygen removing step there is a tendency to entrainair bubbles in the solution. Where a HMDE is used there is a fear thatair bubbles may deposit on the mercury drop or the mercury drop may falldown. For this reason, the preferred number of revolutions of thestirrer during the oxygen removing step is generally from 300 to 800RPM, more preferably from 300 to 500 RPM although it depends upon theshape and size of the stirrer and the size of the electrolytic cell.

During the plating step, the tendency of entraining air bubbles is smalland the detection sensitivity increases with the stirring speed.Accordingly, a preferred number of revolutions of the stirrer during theplating step is higher than 1,000 RPM, more preferably higher than 1,500RPM. Although high stirrer speed is preferred, the upper limit ofconventional stirrers is about 2,000 RPM. Where a HMDE is used there isa possibility that the mercury drop may be caused to fall down by thevortex created by high speed stirring, so that it is advantageous toprovide a buffle plate in the electrolytic cell to prevent generation ofthe vortex.

Of course, the switching between the numbers of revolutions of thestirrer during the oxygen removing step and during the plating step canbe done manually, but it can also be done automatically by utilizing asequence circuit combined with a time setting circuit as is well knownin the art.

As described above, a reference electrode is usually disposed in theelectrolytic cell of the apparatus of this invention. In such case, aso-called H type electrolyte cell is used wherein a working electrodecell containing a working electrode and a counter electrode, and areference electrode cell containing a reference electrode areinterconnected through a salt bridge. Usually such H type cell is madeof glass and can be advantageously used in this invention.

However, in the H type cell made of glass it is difficult to make shortthe length of the salt bridge, e.g. less than several centimeters. As aresult, the salt bridge causes a voltage drop causing increase in thenon-faradic current as well as decrease in the measuring accuracy.Further, ions, for example chlorine ions, in the reference electrodecell migrate into the working electrode cell through the salt bridgethereby interferring with the measurement. These defects can be obviatedby using a measuring cell having the following construction.

More particularly, such measuring cell comprises an electrolyte solutioncell disposed between the working electrode cell and the referenceelectrode cell, and wherein these cells are integrally formed in ablock.

FIGS. 15 through 20 show one example of the measuring cell having aconstruction just described, wherein 101 shows the main body of thecell, 102 a working electrode cell, 103 a reference electrode cell, 104an electrolyte solution cell, and 117 and 123 liquid communicatingpassages.

More particularly, the main body 101 is a block made of plastics, glass,metal or the like. From the standpoint of inspection of the interior andeasiness of working it is advantageous to fabricate it with atransparent plastic, for example acrylic resin.

The working electrode cell 102 takes the form of a cylindrical borevertically extending through the main body 101 and provided at thebottom with a sample inlet port 112 and a discharge port 106 for theliquid to be measured. Along the cylindrical bore are provided anopening 111 for inserting the counter electrode, an inert gas inlet port113, a standard sample pouring port 110 and a sample overflow port 114.

The working electrode 120 is inserted into the working electrode cell102 through the opening at the upper end 105 thereof. Preferably theaxis of the end portion 105 is slightly eccentric with respect to theaxis of the working electrode cell. During the measuring operation theliquid being measured 121 contained in the working electrode cell 102 isstirred by a rotor 122 located at the bottom of the cell. Air bubblesformed by the entrained air have a tendency to gather at the center ofthe cell. Consequently, if the working electrode 120 were positioned atthe axial center of the working electrode cell 102 air bubbles woulddeposit on the electrode surface, thus decreasing the measuringsensitivity. However, when the working electrode is positioned slightlyeccentrically as above described, deposition of the air bubbles can beprevented efficiently. Moreover, as the linear velocity of the liquidbecomes higher at the peripheral portion than at the central portion,the measuring accuracy is improved. When a lens shaped projection 119 isprovided on the outer surface of the main body 101 at a positioncorresponding to that of the working electrode 120 it is advantageous toobserve the condition of the working electrode.

The reference electrode cell 103 and the electrolyte solution cell 104take the form of cylindrical bores parallel with the working electrodecell 102. The bottoms of the cells 103 and 104 are intercommunicatedthrough a liquid communicating passage 123, whereas the electrolytesolution cell 104 is communicated with the working electrode cell 102through a communication passage 117 as shown in FIG. 17.

An electrode liquid is filled in the reference electrode cell 103 andthe reference electrode is hung through its upper opening.

The liquid communication passages 117 and 123 are formed to open to theouter surface of the main body 101 and provided with partition walls 118and 116 respectively made of ceramic, porous fluorinated polymer oragaragar for separating intercommunicated cells. The joint between thecommunication passage 123 and the electrolyte solution cell 104 islocated at a lower level than the joint between the passage 117 and theelectrolyte solution cell 104. Preferably, the length of the liquidcommunication passages 117 and 123 is made to be short, for example 5 to30 mm, preferably 5 to 20 mm. The outer openings of these passages areclosed by plugs, not shown.

Although it is preferred to charge in the electrolyte solution cell 104the same electrolyte as the supporting electrolyte for the solutionbeing measured, usually, however, an aqueous solution of potassiumchloride or potassium nitrate, is used.

Admission of the solution being measured into the working electrode cell102 is made by continuously admitting the solution through inlet port112, causing it to overflow, and then stopping the admission when theinside of the cell is throughly washed and replaced by the solution.

After completion of the measurement, the solution is discharged into awaste tank 107 through discharge opening 106, and a conduit (not shown)between the same and an opening 115 leading to the tank 107 by operatinga suction pump (not shown) mounted in the upper opening of the wastetank 107 which is in parallel with the working electrode cell 102. Thenthe waste solution is discharged to the outside through the upperopening of the waste cell 107.

Where a mercury electrode is used as the working electrode, mercury isseparated in the waste tank 107 and discharged through discharge port108.

In the measuring cell described above, since the length of the saltbridge is small, not only the nonfaradaic current is small, but also asthe movement of the content between intercommunicated cells does notinterfere with the measurement, the cell is especially suitable formeasuring a small quantity of ions. Moreover, as the entire cell ismanufactured by working a single block, it has a large mechanicalstrength and is easy to handle.

We claim:
 1. Differential pulse anodic stripping voltammetry apparatuscomprising:an electrolytic cell having a working electrode and a counterelectrode; a source of plating voltage, a source of sweeping strippingvoltage and a source of pulse voltage for impressing plating voltage,stripping voltage and pulse voltage respectively across the workingelectrode and the counter electrode; a current-voltage converter forconverting the current flowing through the working electrode intovoltage for producing two sample voltages; two voltage holding meanseach having a filter circuit with a time constant of 0.01 to 0.1 secondfor holding the two sample voltages; and a differential amplifier foramplifying the difference between the two sample voltages held by thevoltage holding means.
 2. The apparatus according to claim 1 wherein thetime constant of the filter circuit is 0.01 to 0.08 second.
 3. Theapparatus according to claim 1 wherein the time constant of the filtercircuit is 0.01 to 0.05 second.
 4. The apparatus according to claim 1which further comprises means for discharging voltage held by thevoltage holding means after completion of measurement.